A form of numerically controlled machining is one in which there is an area bounded by a closed curve, wherein surface cutting is applied to a portion (convex portion) projecting from an area on the outer side of the closed curve.
Such an area cutting (surface cutting) method includes the following steps:
(a) inputting data specifying a closed curve CCL of an area AR shown in FIG. 6, cutting direction (direction of arrow A) along cutting path PT.sub.i (i=1, 2, . . . ), shift direction (direction of arrow B) in which a tool TL is shifted by a predetermined amount of cut-in whenever surface cutting along the cutting path PT.sub.i ends, and cut-in amount P;
(b) generating the cutting path PT.sub.i on the basis of the inputted data;
(c) performing cutting by moving the tool along the cutting path PT.sub.i from a cutting starting point P.sub.i to a cutting end point Q.sub.i on the generated cutting path PT.sub.i ;
(d) obtaining the next cutting path PT.sub.i+1, which results when the tool is shifted by the amount of cut-in P following the end of cutting along the preceding cutting path;
(e) thereafter performing cutting (unidirectional cutting) by moving the tool from point P.sub.i+1 to point Q.sub.i+1, in which point P.sub.i+1 is taken as the cutting starting point of cutting path PT.sub.i+1 and point Q.sub.i+1 is taken as the cutting end point of cutting path PT.sub.i+1, or performing cutting (back-and-forth cutting) by moving the tool from point Q.sub.i+1 to point P.sub.i+1, in which point Q.sub.i+1 is taken as the cutting starting point of cutting path PT.sub.i+1 and point P.sub.i+1 is taken as the cutting end point of cutting path PT.sub.i +1 ; and
(f) repeating the unidirectional or back-and-forth cutting operation from this point onward to surface-cut the area AR.
In the conventional surface cutting method, the positions of the cutting starting point and cutting end point of each cutting path PTi are set appropriately. This results in a long tool pass, namely in a lengthy period of time during which surface cutting is not carried out. Consequently, machining efficiency is poor.
In addition, since the actual cut-in amount P is decided appropriately in the prior art, the amount of cut-in is too small and results in a large number of cutting strokes (cutting paths) and diminished cutting efficiency, or the amount of cut-in may be non-uniform (e.g. the last cut-in being very small in comparison with the others).
Accordingly, an object of the present invention is to provide a surface cutting method through which tool pass can be shortened, the number of cutting strokes reduced and the amount of cut-in made uniform.